The present invention relates to the field CMOS technology voltage controlled oscillators. More particularly, the present invention relates to a high speed CMOS voltage controlled oscillator (VCO) for digital communications.
Voltage controlled oscillators (VCOs) are well known and widely used in the electronics industry. Within the digital communications field, VCOs are used in a variety of applications. Such applications include, for example, frequency synthesizers, signal generation, (e.g., serial transmission clock recovery) and the like. VCOs are typically designed to perform within a given set of boundary conditions and to perform according to a specified standard. Typical conditions include, for example, performance over operating temperature ranges, sensitivity to vibration, output sensitivity to interference, and the like. Typical performance standards include, for example, output signal frequency stability, output signal programmability, and the like.
A typical prior art VCO circuit generates an oscillating output signal having a specified frequency. The signal can have several different wave forms (e.g., square, saw tooth, triangular, etc.). The frequency of the output is tunable and is a function of an input voltage, an external resistance or capacitance, or the like. The type of application in which the VCO circuit is used dictates its operating conditions and performance requirements.
In addition, the type of application also largely determines type of fabrication technology used to manufacture the VCO. A large number of modern digital integrated circuits are fabricated using well known and widely used CMOS technology. Where the VCO circuit is included in a CMOS IC (integrated circuit), it is usually fabricated in CMOS (e.g., fabricated using CMOS process technology).
There is a problem, however, when the application in which the overall IC is used requires the VCO circuit to have an output with very low phase noise. For example, where the IC is part of a high speed serial transmission system (e.g., high speed wireless transmission systems) it is important that the output frequency of the VCO circuit be stable and jitter free, and be a consistent function of the control inputs (e.g., voltage, capacitance, and the like) while the output frequency exhibits extremely low phase noise.
For example, in a case where a prior art VCO circuit is used in an application for clock recovery in a gigabit serial transmission system, it is important that the output frequency remain stable and jitter free, and the output waveform remain within specified limits, even at the output frequencies of 1 GHz or more. The output frequency is used to reconstruct a serial transmission clock signal, which in turn, is used to sample data on a serial transmission line. Very little phase noise on the output signal can be tolerated. Distortion, defects, irregularity, or variation in the VCO output frequency or the waveform can have a very detrimental effect on the reconstructed clock signal, and hence, could lead to sampling errors, lost data, decreased throughput, or other such problems.
Consequently, for these very high performance applications it is important that the VCO circuit provide a very stable, jitter free output signal at the specified frequency, and that the output frequency exhibit as little phase noise is possible. However, prior art CMOS VCOs cannot reliably function at such high frequencies. Prior art CMOS VCOs cannot reliably generate output signals having an acceptable waveform (e.g., free of phase noise) and having acceptable stability.
Thus, what is required is a CMOS VCO circuit which solves the low phase noise operation problems of the prior art. What is required is a circuit capable of reliable operation at high frequencies while exhibiting very low phase noise on the output signal. What is required is a circuit which produces a stable, jitter free output signal with a waveform free of defects and irregularities. The present invention provides an advantageous solution to the above requirements.
The present invention provides a CMOS VCO circuit which solves the low phase noise operation problems of the prior art. The present invention provides a circuit capable of reliable operation at high frequencies while exhibiting very low phase noise on the output signal. The circuit of the present invention produces a stable, jitter free output signal with a waveform free of defects and irregularities.
In one embodiment, the present invention is implemented as a low phase noise CMOS voltage controlled oscillator (VCO) circuit. In this implementation, the VCO circuit includes a bias circuit and at least one VCO cell coupled to the bias circuit. The bias circuit is configured to produce a bias output. The VCO cell (or series of identical VCO cells) includes a VCO output for transmitting a VCO output signal. A frequency to voltage converter is coupled to receive the VCO output signal. The frequency to voltage converter converts a frequency of the VCO output signal into a corresponding voltage output. The voltage output is coupled to control the bias circuit. The VCO cell also includes a current source coupled to the bias circuit such that the voltage output from the voltage a current converter provides negative feedback to the VCO cell via the current source. The bias output from the bias circuit controls the amount of current flowing through the VCO cell, and thus, the frequency of the VCO output signal. The negative feedback, in turn, reduces the phase noise on the VCO output signal. In this manner, the VCO circuit produces a stable, jitter free VCO output signal with a waveform free of defects and irregularities.